Multi-layered magnetic pigments and foils

ABSTRACT

Multilayered magnetic pigment flakes and foils are provided. The pigment flakes can have a symmetrical coating structure on opposing sides of a magnetic core, or can be formed with encapsulating coatings around the magnetic core. The magnetic core can be a magnetic layer between reflector or dielectric layers, a dielectric layer between magnetic layers, or only a magnetic layer. Some embodiments of the pigment flakes and foils exhibit a discrete color shift so as to have distinct colors at differing angles of incident light or viewing. The pigment flakes can be interspersed into liquid media such as paints or inks to produce colorant compositions for subsequent application to objects or papers. The foils can be laminated to various objects or can be formed on a carrier substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. Ser. No. 09/844,261, filed Apr.27, 2001, which application is incorporated herein by specificreference, and claims priority thereto.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to pigments and foils. Inparticular, the present invention relates to multilayered pigment flakesand foils which have magnetic layers, and pigment compositions thatincorporate multilayer pigment flakes having magnetic layers.

2. The Relevant Technology

Various pigments, colorants, and foils have been developed for a widevariety of applications. For example, magnetic pigments have beendeveloped for use in applications such as decorative cookware, creatingpatterned surfaces, and security devices. Similarly, color shiftingpigments have been developed for such uses as cosmetics, inks, coatingmaterials, ornaments, ceramics, automobile paints, anti-counterfeitinghot stamps, and anti-counterfeiting inks for security documents andcurrency.

Color shifting pigments, colorants, and foils exhibit the property ofchanging color upon variation of the angle of incident light, or as theviewing angle of the observer is shifted. The color-shifting propertiesof pigments and foils can be controlled through proper design of theoptical thin films or orientation of the molecular species used to formthe flake or foil coating structure. Desired effects can be achievedthrough the variation of parameters such as thickness of the layersforming the flakes and foils and the index of refraction of each layer.The changes in perceived color which occur for different viewing anglesor angles of incident light are a result of a combination of selectiveabsorption of the materials comprising the layers and wavelengthdependent interference effects. The interference effects, which arisefrom the superposition of light waves that have undergone multiplereflections, are responsible for the shifts in color perceived withdifferent angles. The reflection maxima changes in position andintensity, as the viewing angle changes, due to changing interferenceeffects arising from light path length differences in the various layersof a material which are selectively enhanced at particular wavelengths.

Various approaches have been used to achieve such color shiftingeffects. For example, small multilayer flakes, typically composed ofmultiple layers of thin films, are dispersed throughout a medium such aspaint or ink that may then be subsequently applied to the surface of anobject. Such flakes may optionally be overcoated to achieve desiredcolors and optical effects. Another approach is to encapsulate smallmetallic or silicatic substrates with varying layers and then dispersethe encapsulated substrates throughout a medium such as paint or ink.Additionally, foils composed of multiple layers of thin films on asubstrate material have been made.

One manner of producing a multilayer thin film structure is by formingit on a flexible web material with a release layer thereon. The variouslayers are deposited on the web by methods well known in the art offorming thin coating structures, such as PVD, sputtering, or the like.The multilayer thin film structure is then removed from the web materialas thin film color shifting flakes, which can be added to a polymericmedium such as various pigment vehicles for use as an ink or paint. Inaddition to the color shifting flakes, additives can be added to theinks or paints to obtain desired color shifting results.

Color shifting pigments or foils are formed from a multilayer thin filmstructure that includes the same basic layers. These include an absorberlayer(s), a dielectric layer(s), and optionally a reflector layer, invarying layer orders. The coatings can be formed to have a symmetricalmultilayer thin film structure, such as:

absorber/dielectric/reflector/dielectric/absorber; orabsorber/dielectric/absorber.

Coatings can also be formed to have an asymmetrical multilayer thin filmstructure, such as:

absorber/dielectric/reflector.

For example, U.S. Pat. No. 5,135,812 to Phillips et al., which isincorporated by reference herein, discloses color-shifting thin filmflakes having several different configurations of layers such astransparent dielectric and semi-transparent metallic layered stacks. InU.S. Pat. No. 5,278,590 to Phillips et al., which is incorporated byreference herein, a symmetric three layer optical interference coatingis disclosed which comprises first and second partially transmittingabsorber layers which have essentially the same material and thickness,and a dielectric spacer layer located between the first and secondabsorber layers.

Color shifting platelets for use in paints are disclosed in U.S. Pat.No. 5,571,624 to Phillips et al., which is incorporated by referenceherein. These platelets are formed from a symmetrical multilayer thinfilm structure in which a first semi-opaque layer such as chromium isformed on a substrate, with a first dielectric layer formed on the firstsemi-opaque layer. An opaque reflecting metal layer such as aluminum isformed on the first dielectric layer, followed by a second dielectriclayer of the same material and thickness as the first dielectric layer.A second semi-opaque layer of the same material and thickness as thefirst semi-opaque layer is formed on the second dielectric layer.

With regard to magnetic pigments, U.S. Pat. No. 4,838,648 to Phillips etal. (hereinafter “Phillips '648”) discloses a thin film magnetic colorshifting structure wherein the magnetic material can be used as thereflector or absorber layer. One disclosed magnetic material is a cobaltnickel alloy. Phillips '648 discloses flakes and foils with thefollowing structures:

dyed superstrate/absorber/dielectric/magnetic layer/substrate; dyedsuperstrate/absorber/dielectric/magnetic layer/dielectric/absorber/dyedsuperstrate; and

adhesive/magnetic layer/dielectric/absorber/releasablehardcoat/substrate.

Patterned surfaces have been provided by exposing magnetic flakes to amagnetic force to effect a physical alteration in the structure of thepigment. For example, U.S. Pat. No. 6,103,361 to Batzar et al.(hereinafter “Batzar”) uses pigments made of magnetizable materials todecorate cookware. In particular, Batzar is directed toward controllingthe orientation of stainless steel flakes in a fluoropolymer releasecoating to make patterns where at least some of the flakes are longerthan the coating thickness. The patterned substrate is formed byapplying magnetic force through the edges of a magnetizable diepositioned under a coated base to alter the orientation of the flakeswithin the coating, thereby inducing an imaging effect or pattern.However, Batzar does not discuss the use of optical thin film stacks orplatelets employing a magnetic layer. In addition, although thestainless steel flakes used in Batzar are suitable for decoratingcookware, they are poorly reflecting.

U.S. Pat. No. 2,570,856 to Pratt et al (hereinafter “Pratt”) is directedto metallic flake pigments which are based on ferromagnetic metalplatelets. Like Batzar, however, Pratt uses poorly reflecting metals anddoes not teach the use of thin film optical stacks.

U.S. Pat. Nos. 5,364,689 and 5,630,877 to Kashiwagi et al., (hereinaftercollectively “the Kashiwagi patents”), the disclosures of which areincorporated herein by reference, disclose methods and apparatus forcreating magnetically formed painted patterns. The Kashiwagi patentsteach use of a magnetic paint layer, which includes non-sphericalmagnetic particles in a paint medium. A magnetic field with magneticfield lines in the shape of the desired pattern is applied to the paintlayer. The final pattern is created by the different magnetic particleorientations in the hardened paint.

One attempt at incorporating a magnetic layer into a multilayer flake isdisclosed in European Patent Publication EP 686675B1 to Schmid et al.(hereinafter “Schmid”), which describes laminar color shiftingstructures which include a magnetic layer between the dielectric layerand a central aluminum layer as follows:

oxide/absorber/dielectric/magnet/Al/magnet/dielectric/absorber/oxide

Thus, Schmid uses aluminum platelets and then coats these platelets withmagnetic materials. However, the overlying magnetic material downgradesthe reflective properties of the pigment because aluminum is the secondbrightest metal (after silver), meaning any magnetic material is lessreflective. Further, Schmid starts with aluminum platelets generatedfrom ballmilling, a method which is limited in terms of the layersmoothness that can be achieved.

Patent Publication EP 710508A1 to Richter et al. (hereinafter “Richter”)discloses methods for providing three dimensional effects by drawingwith magnetic tips. Richter describes three dimensional effects achievedby aligning magnetically active pigments in a spatially-varying magneticfield. Richter uses standard pigments (barium ferrite, strontiumferrite, samarium/cobalt, Al/Co/Ni alloys, and metal oxides made bysintering and quick quenching, none of which are composed of opticalthin film stacks. Rather, the particles are of the hard magnetic type.Richter uses electromagnetic pole pieces either on top of the coating oron both sides of the coating. However, Richter uses a moving system andrequires “drawing” of the image. This “drawing” takes time and is notconducive to production type processes.

U.S. Pat. No. 3,791,864 to Steingroever (hereinafter “Steingroever”)describes a method for patterning magnetic particles by orienting themwith a magnetic pattern generated in an underlying prime coating thathas previously been patterned by a magnetic field. The prime coatcontains magnetic particles of the type MO×6Fe₂O₃ where M can be one ormore of the elements Ba, Sr, Co, or Pb. After coating a continuous sheetof liquid coating of the primer, it is hardened and then areas of theprimer are magnetized by a magnetic field. Next, a pigment vehicle withmagnetic particles suspended therein is then applied. The magneticparticles suspended therein are finally oriented by the magnetic forcefrom the magnetic pattern in the primer, creating the final pattern.However, Steingroever suffers from a diffuse magnetic image in the primecoat, which in turn passes a diffuse image to the topcoat. Thisreduction in resolution is because high magnetic fields are limited inthe resolution they can create. This limitation is due to high magneticfield lines surrounding the intended magnetic image, thereby affectinguntargeted magnetic particles in the prime coat and blurring the image.

Accordingly, there is a need for improved multilayer pigment flakes andfoils with magnetic properties that overcome or avoid the above problemsand limitations.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the invention to provide durable magnetic flakes andfoils.

It is another object of the invention to provide magnetic color shiftingflakes and foils with high chroma.

It is a further object of the invention to provide pigment flakes andfoils with security features that are not visually perceptible.

It is yet another object of the invention to provide pigment flakes andfoils capable of providing three dimensional like images.

To achieve the foregoing objects and in accordance with the invention asembodied and broadly described herein, pigment flakes and foils areprovided which have magnetic properties. The pigment flakes can have asymmetrical stacked coating structure on opposing sides of a magneticcore layer, can have an asymmetrical coating structure with all of thelayers on one side of the magnetic layer, or can be formed with one ormore encapsulating coatings around a magnetic core. The coatingstructure of the flakes and foils includes at least one magnetic layerand optionally one or more of a reflector layer, dielectric layer, andabsorber layer. In color shifting embodiments of the invention, thecoating structure includes the dielectric layer overlying the magneticand reflector layers, and the absorber layer overlying the dielectriclayer. Non color shifting embodiments of the invention include amagnetic layer between two reflector layers or encapsulated by areflector layer, a magnetic layer between two dielectric layers orencapsulated by a dielectric layer, a dielectric layer between twomagnetic layers or encapsulated by a magnetic layer, and a magneticlayer encapsulated by a colorant layer.

The color shifting embodiments exhibit a discrete color shift so as tohave a first color at a first angle of incident light or viewing and asecond color different from the first color at a second angle ofincident light or viewing. The pigment flakes can be interspersed intoliquid media such as paints or inks to produce colorant compositions forsubsequent application to objects or papers. The foils can be laminatedto various objects or can be formed on a carrier substrate.

These and other objects and features of the present invention willbecome more fully apparent from the following description and appendedclaims, or may be learned by the practice of the invention as set forthhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to illustrate the manner in which the above-recited and otheradvantages and features of the invention are obtained, a more particulardescription of the invention briefly described above will be rendered byreference to specific embodiments thereof which are illustrated in theappended drawings. Understanding that these drawings depict only typicalembodiments of the invention and are not therefore to be consideredlimiting of its scope, the invention will be described and explainedwith additional specificity and detail through the use of theaccompanying drawings in which:

FIG. 1 is a schematic representation of the coating structure of amagnetic flake according to one embodiment of the invention;

FIG. 2 is a schematic representation of the coating structure of amagnetic flake according to another embodiment of the invention;

FIG. 3 is a schematic representation of the coating structure of amagnetic flake according to an alternative embodiment of the invention;

FIG. 4 is a schematic representation of the coating structure of amagnetic flake according to another embodiment of the invention;

FIG. 5 is a schematic representation of the coating structure of amagnetic flake according to a further embodiment of the invention;

FIG. 6 is a schematic representation of the coating structure of amagnetic flake according to a further embodiment of the invention;

FIG. 7 is a schematic representation of the coating structure of amagnetic flake according to an alternative embodiment of the invention;

FIG. 8 is a schematic representation of the coating structure of amagnetic flake according to a further embodiment of the invention;

FIG. 9 is a schematic representation of the coating structure of amagnetic flake according to yet a further embodiment of the invention;

FIG. 10 is a schematic representation of the coating structure of amagnetic flake according to another alternative embodiment of theinvention;

FIG. 11 is a schematic representation of the coating structure of amagnetic flake according to another embodiment of the invention;

FIG. 12 is a schematic representation of the coating structure of amagnetic flake according to a further embodiment of the invention;

FIG. 13 is a schematic representation of the coating structure of amagnetic foil according to one embodiment of the invention;

FIG. 14 is a schematic representation of the coating structure of amagnetic foil according to another embodiment of the invention;

FIG. 15 is a schematic representation of the coating structure of amagnetic foil according to a further embodiment of the invention;

FIG. 16 is a schematic representation of the coating structure of anoptical article according to an additional embodiment of the invention;and

FIG. 17 is a schematic representation of the coating structure of anoptical article according to a further embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to multilayer pigment flakes and foilswhich have magnetic layers, and pigment compositions which incorporatethe magnetic flakes. The flakes and foils can be used both to createsecurity features which are not visually perceptible, and to createthree dimensional-like images for security devices or to add decorativefeatures to a product. The nonvisual security features are provided byburying the magnetic layer between other layers within a flake or foilso that only the overlying layers are exposed.

The three dimensional-like effects can be provided by exposing the flakeor foil to an external magnetic force, thereby orienting the plane ofsome of the pigments normal to the surface of the coating. Theun-oriented pigments lie with their planar surface parallel to thesurface of the coating. The three dimensional-like effect is due to thealignment of the particles such that the aspect ratio is oriented withthe magnetic field, i.e. the longest part of the pigment aligns itselfalong the magnetic field lines. In such case, the face of the pigment isturned away from the observer to various extents depending on themagnitude of the magnetic force. In the limit or maximum orientation,the coating appears black in color. As one moves off the black, onemoves slowly toward the color of the planar surface of the pigment,i.e., color shifting, non-color shifting, such as the color blue, orsilver as for example, aluminum. The result is a colored threedimensional-like effect, similar to that of a holographic effect, thatappears to move as the viewing angle changes. Methods of creating threedimensional-like images using the magnetic pigments disclosed herein aredescribed in further detail in a copending U.S. patent application, andentitled Methods For Producing Imaged Coated Articles By Using MagneticPigments, the disclosure of which is incorporated herein by reference.

Unlike many prior magnetic flakes, the presently disclosed flakes arenot composed only of magnetizable materials, but include bothmagnetizable and non-magnetizable materials. For example, the inventionencompasses pigment flakes wherein a magnetic layer is buried within oneor more reflector layers. In another embodiment the pigment flakescomprise a magnetic core surrounded by dielectric layers. In yet afurther embodiment, the pigment flakes include a dielectric coresurrounded by magnetic layers.

In the case of magnetic layers buried between or within overlyingreflector layers, the present invention presents a significantimprovement over the prior art by substantially achieving higher chromaand brightness. By putting the duller magnetic material inside thereflector, the present invention accomplishes two objectives: 1) thereflectivity of the reflector layer is maintained; and 2) color shiftingpigments without the inner core of magnetic material cannot bedistinguished by an observer from such pigment with the core of magneticmaterial. For example, two coated objects viewed side by side, one withand one without the magnetic material in the coating, would look thesame to the observer. However, the magnetic color shifting pigmentprovides a covert security feature in addition to the color shiftingeffect. Thus, with a magnetic detection system, a magnetic covertsignature in the pigment could be read by a Faraday rotator detector,for example.

In various embodiments of the present invention, the pigment flakes andfoils have substantial shifts in chroma and hue with changes in angle ofincident light or viewing angle of an observer. Such an optical effect,known as goniochromaticity or “color shift,” allows a perceived color tovary with the angle of illumination or observation. Accordingly, suchpigment flakes and foils exhibit a first color at a first angle ofincident light or viewing and a second color different from the firstcolor at a second angle of incident light or viewing. The pigment flakescan be interspersed into liquid media such as paints or inks to producevarious color shifting colorant compositions for subsequent applicationto objects or papers. The foils can be laminated to various objects orcan be formed on a carrier substrate.

Generally, the color shifting pigment flakes can have a symmetricalstacked coating structure on opposing sides of a magnetic core layer,can have an asymmetrical coating structure with a majority of the layerson one side of the magnetic layer, or can be formed with one or moreencapsulating coatings which surround a magnetic core. The coatingstructure of the flakes and foils generally includes a magnetic core,which includes a magnetic layer and other optional layers, a dielectriclayer overlying the magnetic core, and an absorber layer overlying thedielectric layer.

The color shifting flakes and foils of the invention can be formed usingconventional thin film deposition techniques, which are well known inthe art of forming thin coating structures. Nonlimiting examples of suchthin film deposition techniques include physical vapor deposition (PVD),chemical vapor deposition (CVD), plasma enhanced (PE) variations thereofsuch as PECVD or downstream PECVD, sputtering, electrolysis deposition,and other like deposition methods that lead to the formation of discreteand uniform thin film layers.

The color shifting pigment flakes of the invention can be formed byvarious fabrication methods. For example, the pigment flakes can beformed by a web coating process in which various layers are sequentiallydeposited on a web material by conventional deposition techniques toform a thin film structure, which is subsequently fractured and removedfrom the web, such as by use of a solvent, to form a plurality of thinfilm flakes.

In another fabrication method, one or more thin film layers including atleast the magnetic layer is deposited on a web to form a film, which issubsequently fractured and removed from the web to form a plurality ofpigment preflakes. The preflakes can be fragmented further by grindingif desired. The preflakes are then coated with the remaining layer orlayers in a sequential encapsulation process to form a plurality ofpigment flakes. A similar process is disclosed in further detail incopending U.S. application Ser. No. 09/512,116, filed on Feb. 24, 2000,the disclosure of which is incorporated by reference herein.

In another fabrication method, magnetic particles can be coated in asequential encapsulation process to form a plurality of pigment flakes.When an encapsulation process is used for forming the outer layers ofthe flakes, it will be appreciated that each respective encapsulatinglayer is a continuous layer composed of one material and havingsubstantially the same thickness around the flake structure. In someembodiments of the invention, the encapsulating layer can be a coloreddielectric material or an organic layer with added colorant.

Referring now to the drawings, wherein like structures are provided withlike reference designations, the drawings only show the structuresnecessary to understand the present invention. FIG. 1 depicts areflective magnetic flake (“RMF”) 20 according to one embodiment of theinvention. The RMF 20 is a three layer design having a generallysymmetrical thin film structure with a central magnetic layer 22 and atleast one reflector layer on either or both of the opposing majorsurfaces of the central magnetic layer. Thus, RMF 20 comprises amagnetic layer interdisposed between a reflector layer 24 and anopposing reflector layer 26. By inserting the magnetic layer between thehighly reflective reflector layers, such as aluminum, the opticalproperties of the reflector layers are not degraded and the flakeremains highly reflective. The RMF 20 can be used as a pigment flake orcan be used as a core section with additional layers applied thereoversuch as in a color shifting pigment. In the case of color shiftingpigments, maintaining the high reflective layer is extremely importantto preserve high brightness and chroma. Each of these layers in thecoating structure of RMF 20 is discussed below in greater detail.

The magnetic layer 22 can be formed of any magnetic material such asnickel, cobalt, iron, gadolinium, terbium, dysprosium, erbium, and theiralloys or oxides. For example, a cobalt nickel alloy can be employed,with the cobalt and nickel having a ratio by weight of about 80% andabout 20%, respectively. This ratio for each of these metals in thecobalt nickel alloy can be varied by plus or minus about 10% and stillachieve the desired results. Thus, cobalt can be present in the alloy inan amount from about 70% to about 90% by weight, and nickel can bepresent in the alloy in an amount from about 10% to about 30% by weight.Other examples of alloys include Fe/Si, Fe/Ni, FeCo, Fe/Ni/Mo, andcombinations thereof. Hard magnetics of the type SmCo₅, NdCo₅, Sm₂Co₁₇,Nd₂Fe₁₄B, Sr₆Fe₂O₃, TbFe₂, Al—Ni—Co, and combinations thereof, can alsobe used as well as spinel ferrites of the type Fe₃O₄, NiFe₂O₄, MnFe₂O₄,CoFe₂O₄, or garnets of the type YIG or GdIG, and combinations thereof.The magnetic material may be selected for its reflecting or absorbingproperties as well as its magnetic properties. When utilized to functionas a reflector, the magnetic material is deposited to a thickness sothat it is substantially opaque. When utilized as an absorber, themagnetic material is deposited to a thickness so that it is notsubstantially opaque. A typical thickness for the magnetic material whenutilized as an absorber is from about 2 nm to about 20 nm.

Although this broad range of magnetic materials can be used, the “soft”magnets are preferred in some embodiments of the invention. As usedherein, the term “soft magnets” refers to any material exhibitingferromagnetic properties but having a remanence that is substantiallyzero after exposure to a magnetic force. Soft magnets show a quickresponse to an applied magnetic field, but have very low (coercivefields (Hc)=0.05-300 Oersteds (Oe)) or zero magnetic signatures, orretain very low magnetic lines of force after the magnetic field isremoved. Similarly, as used herein, the term “hard magnets” (also calledpermanent magnets) refers to any material that exhibits ferromagneticproperties and that has a long lasting remanence after exposure to amagnetizing force. A ferromagnetic material is any material that has apermeability substantially greater than 1 and that exhibits magnetichysteresis properties.

Preferably, the magnetic materials used to form magnetic layers in theflakes and foils of the invention have a coercivity of less than about2000 Oe, more preferably less than about 300 Oe. Coercivity refers tothe ability of a material to be demagnetized by an external magneticfield. The higher the value of coercivity, the higher the magnetic fieldrequired to de-magnetize the material after the field is removed. Insome embodiments of the invention, the magnetic layers used arepreferably “soft” magnetic materials (easily demagnetized), as opposedto “hard” magnetic materials (difficult to demagnetize) which havehigher coercivities. The coercivities of the foils, pigments orcolorants of the magnetic color shifting designs according to theinvention are preferably in a range of about 50 Oe to about 300 Oe.These coercivities are lower than in standard recording materials. Thus,preferred embodiments of the invention which use soft magnets inmagnetic color shifting pigments and magnetic non color shiftingpigments are an improvement over conventional technologies. The use ofsoft magnetic materials in pigment flakes allows for easier dispersionof the flakes without clumping.

The magnetic layer 22 can be formed to have a suitable physicalthickness of from about 200 angstroms (Å) to about 10,000 Å, andpreferably from about 500 Å to about 1,500 Å. However, it will beappreciated by those skilled in the art, in view of the disclosureherein, that the optimal magnetic thickness will vary depending on theparticular magnetic material used and the purpose for its use. Forexample, a magnetic absorber layer will be thinner than a magneticreflector layer based on the optical requirements for such layers, whilea covert magnetic layer will have a thickness based solely on itsmagnetic properties.

The reflector layers 24 and 26 can be composed of various reflectivematerials. Presently preferred materials are one or more metals, one ormore metal alloys, or combinations thereof, because of their highreflectivity and ease of use, although non-metallic reflective materialscould also be used. Nonlimiting examples of suitable metallic materialsfor the reflector layers include aluminum, silver, copper, gold,platinum, tin, titanium, palladium, nickel, cobalt, rhodium, niobium,chromium, and combinations or alloys thereof. These can be selectedbased on the f desired. The reflector layers 24, 26 can be formed tohave a suitable physical thickness of from about 400 Å to about 2,000 Å,and preferably from about 500 Å to about 1,000 Å.

In an alternative embodiment, opposing dielectric layers may optionallybe added to overlie reflector layers 24 and 26. These opposingdielectric layers add durability, rigidity, and corrosion resistance toRMF 20. Alternatively, an encapsulating dielectric layer may be formedto substantially surround reflector layers 24, 26 and magnetic layer 22.The dielectric layer(s) may be optionally clear, or may be selectivelyabsorbing so as to contribute to the color effect of the pigment flake.Examples of suitable dielectric materials for the dielectric layers aredescribed hereafter.

FIG. 2 depicts a magnetic color shifting pigment flake 40 based upon aRMF according to one embodiment of the invention. The flake 40 is agenerally symmetrical multilayer thin film structure having layers onopposing sides of a RMF 42. Thus, first and second dielectric layers 44and 46 are disposed respectively on opposing sides of RMF 42, and firstand second absorber layers 48 and 50 are disposed respectively on eachof dielectric layers 44 and 46. The RMF is as discussed hereinabove forFIG. 1 while the dielectric and absorber layers are discussed below ingreater detail.

The dielectric layers 44 and 46 act as spacers in the thin film stackstructure of flake 40. These layers are formed to have an effectiveoptical thickness for imparting interference color and desired colorshifting properties. The dielectric layers may be optionally clear, ormay be selectively absorbing so as to contribute to the color effect ofa pigment. The optical thickness is a well known optical parameterdefined as the product ηd, where η is the refractive index of the layerand d is the physical thickness of the layer. Typically, the opticalthickness of a layer is expressed in terms of a quarter wave opticalthickness (QWOT) that is equal to 4ηd/λ, where λ is the wavelength atwhich a QWOT condition occurs. The optical thickness of dielectriclayers can range from about 2 QWOT at a design wavelength of about 400nm to about 9 QWOT at a design wavelength of about 700 nm, andpreferably 2-6 QWOT at 400-700 nm, depending upon the color shiftdesired. The dielectric layers typically have a physical thickness ofabout 100 nm to about 800 nm, depending on the color characteristicsdesired.

Suitable materials for dielectric layers 44 and 46 include those havinga “high” index of refraction, defined herein as greater than about 1.65,as well as those have a “low” index of refraction, which is definedherein as about 1.65 or less. Each of the dielectric layers can beformed of a single material or with a variety of material combinationsand configurations. For example, the dielectric layers can be formed ofonly a low index material or only a high index material, a mixture ormultiple sublayers of two or more low index materials, a mixture ormultiple sublayers of two or more high index materials, or a mixture ormultiple sublayers of low index and high index materials. In addition,the dielectric layers can be formed partially or entirely of high/lowdielectric optical stacks, which are discussed in further detail below.When a dielectric layer is formed partially with a dielectric opticalstack, the remaining portion of the dielectric layer can be formed witha single material or various material combinations and configurations asdescribed above.

Examples of suitable high refractive index materials for the dielectriclayer include zinc sulfide (ZnS), zinc oxide (ZnO), zirconium oxide(ZrO₂), titanium dioxide (TiO₂), diamond-like carbon, indium oxide(In₂O₃), indium-tin-oxide (ITO), tantalum pentoxide (Ta2O5), ceric oxide(CeO₂), yttrium oxide (Y₂O₃), europium oxide (Eu₂O₃), iron oxides suchas (II)diiron(III) oxide (Fe₃O₄) and ferric oxide (Fe₂O₃), hafniumnitride (HfN), hafnium carbide (HfC), hafnium oxide (HfO₂), lanthanumoxide (La₂O₃), magnesium oxide (MgO), neodymium oxide (Nd₂O₃),praseodymium oxide (Pr₆O₁₁), samarium oxide (Sm₂O₃), antimony trioxide(Sb₂O₃), silicon monoxide (SiO), selenium trioxide (Se₂O₃), tin oxide(SnO₂), tungsten trioxide (WO₃), combinations thereof, and the like.

Suitable low refractive index materials for the dielectric layer includesilicon dioxide (SiO₂), aluminum oxide (Al₂O₃), metal fluorides such asmagnesium fluoride (MgF₂), aluminum fluoride (AlF₃), cerium fluoride(CeF₃), lanthanum fluoride (LaF₃), sodium aluminum fluorides (e.g.,Na₃AlF₆ or Na₅Al₃F₁₄), neodymium fluoride (NdF₃), samarium fluoride(SmF₃), barium fluoride (BaF₂), calcium fluoride (CaF₂), lithiumfluoride (LiF), combinations thereof, or any other low index materialhaving an index of refraction of about 1.65 or less. For example,organic monomers and polymers can be utilized as low index materials,including dienes or alkenes such as acrylates (e.g., methacrylate),perfluoroalkenes, polytetrafluoroethylene (Teflon), fluorinated ethylenepropylene (FEP), combinations thereof, and the like.

It should be appreciated that several of the above-listed dielectricmaterials are typically present in non-stoichiometric forms, oftendepending upon the specific method used to deposit the dielectricmaterial as a coating layer, and that the above-listed compound namesindicate the approximate stoichiometry. For example, silicon monoxideand silicon dioxide have nominal 1:1 and 1:2 silicon:oxygen ratios,respectively, but the actual silicon:oxygen ratio of a particulardielectric coating layer varies somewhat from these nominal values. Suchnon-stoichiometric dielectric materials are also within the scope of thepresent invention.

As mentioned above, the dielectric layers can be formed of high/lowdielectric optical stacks, which have alternating layers of low index(L) and high index (H) materials. When a dielectric layer is formed of ahigh/low dielectric stack, the color shift at angle will depend on thecombined refractive index of the layers in the stack. Examples ofsuitable stack configurations for the dielectric layers include LH, HL,LHL, HLH, HLHL, LHLH, or in general (LHL)^(n) or (HLH)^(n), wheren=1-100, as well as various multiples and combinations thereof. In thesestacks, LH, for example, indicates discrete layers of a low indexmaterial and a high index material. In an alternative embodiment, thehigh/low dielectric stacks are formed with a gradient index ofrefraction. For example, the stack can be formed with layers having agraded index low-to-high, a graded index high-to-low, a graded index[low-to-high-to-low]^(n), a graded index [high-to-low-to-high]^(n),where n=1-100, as well as combinations and multiples thereof. The gradedindex is produced by a gradual variance in the refractive index, such aslow-to-high index or high-to-low index, of adjacent layers. The gradedindex of the layers can be produced by changing gases during depositionor co-depositing two materials (e.g., L and H) in differing proportions.Various high/low optical stacks can be used to enhance color shiftingperformance, provide antireflective properties to the dielectric layer,and change the possible color space of the pigments of the invention.

The dielectric layers can each be composed of the same material or adifferent material, and can have the same or different optical orphysical thickness for each layer. It will be appreciated that when thedielectric layers are composed of different materials or have differentthicknesses, the flakes exhibit different colors on each side thereofand the resulting mix of flakes in a pigment or paint mixture would showa new color which is the combination of the two colors. The resultingcolor would be based on additive color theory of the two colors comingfrom the two sides of the flakes. In a multiplicity of flakes, theresulting color would be the additive sum of the two colors resultingfrom the random distribution of flakes having different sides orientedtoward the observer.

The absorber layers 48, 50 of flake 40 can be composed of any absorbermaterial having the desired absorption properties, including materialsthat are uniformly absorbing or non-uniformly absorbing in the visiblepart of the electromagnetic spectrum. Thus, selective absorbingmaterials or nonselective absorbing materials can be used, depending onthe color characteristics desired. For example, the absorber layers canbe formed of nonselective absorbing metallic materials deposited to athickness at which the absorber layer is at least partially absorbing,or semi-opaque. Nonlimiting examples of suitable absorber materialsinclude metallic absorbers such as chromium, aluminum, nickel, silver,copper, palladium, platinum, titanium, vanadium, cobalt, iron, tin,tungsten, molybdenum, rhodium, and niobium, as well as theircorresponding oxides, sulfides, and carbides. Other suitable absorbermaterials include carbon, graphite, silicon, germanium, cermet, ferricoxide or other metal oxides, metals mixed in a dielectric matrix, andother substances that are capable of acting as a uniform or selectiveabsorber in the visible spectrum. Various combinations, mixtures,compounds, or alloys of the above absorber materials may be used to formthe absorber layers of flake 40.

Examples of suitable alloys of the above absorber materials includeInconel (Ni—Cr—Fe), stainless steels, Hastalloys (e.g., Ni—Mo—Fe;Ni—Mo—Fe—Cr; Ni—Si—Cu) and titanium-based alloys, such as titanium mixedwith carbon (Ti/C), titanium mixed with tungsten (Ti/W), titanium mixedwith niobium (Ti/Nb), and titanium mixed with silicon (Ti/Si), andcombinations thereof As mentioned above, the absorber layers can also becomposed of an absorbing metal oxide, metal sulfide, metal carbide, orcombinations thereof. For example, one preferred absorbing sulfidematerial is silver sulfide. Other examples of suitable compounds for theabsorber layers include titanium-based compounds such as titaniumnitride (TiN), titanium oxynitride (TiN_(x)O_(y)), titanium carbide(TiC), titanium nitride carbide (TiN_(x)C_(z)), titanium oxynitridecarbide (TiN_(x)O_(y)C_(z)), titanium silicide (TiSi₂), titanium boride(TiB₂), and combinations thereof. In the case of TiN_(x)O_(y) andTiN_(x)O_(y)C_(z), preferably x=0 to 1, y=0 to 1, and z=0 to 1, wherex+y=1 in TiN_(x)O_(y) and x+y+z=1 in TiN_(x)O_(y)C_(z). ForTiN_(x)C_(z), preferably x=0 to 1 and z=0 to 1, where x+z=1.Alternatively, the absorber layers can be composed of a titanium-basedalloy disposed in a matrix of Ti, or can be composed of Ti disposed in amatrix of a titanium-based alloy.

It will be appreciated by one skilled in the art that the absorber layeralso could be formed of a magnetic material, such as a cobalt nickelalloy. This simplifies the manufacture of the magnetic color shiftingdevice or structure by reducing the number of materials required.

The absorber layers are formed to have a physical thickness in the rangefrom about 30 Å to about 500 Å, and preferably about 50 Å to about 150Å, depending upon the optical constants of the absorber layer materialand the desired peak shift. The absorber layers can each be composed ofthe same material or a different material, and can have the same ordifferent physical thickness for each layer.

In an alternative embodiment of flake 40, an asymmetrical color shiftingflake can be provided which includes a thin film stack structure withthe same layers as on one side of RMF 42 as shown in FIG. 2.Accordingly, the asymmetrical color shifting flake includes RMF 42,dielectric layer 44 overlying RMF 42, and absorber layer 48 overlyingdielectric layer 44. Each of these layers can be composed of the samematerials and have the same thicknesses as described above for thecorresponding layers of flake 40. In addition, asymmetrical colorshifting flakes can be formed by a web coating process such as describedabove in which the various layers are sequentially deposited on a webmaterial to form a thin film structure, which is subsequently fracturedand removed from the web to form a plurality of flakes.

In a further alternative embodiment, flake 40 can be formed without theabsorber layers. In this embodiment, opposing dielectric layers 44 and46 are formed of high/low (H/L) dielectric optical stacks such asdescribed previously. Thus, dielectric layers 44 and 46 can beconfigured such that flake 40 has the coating structures:(HL)^(n)/RMF/(LH)^(n), (LH)^(n)/RMF/(HL)^(n), (LHL)^(n)/RMF/(LHL)^(n),(HLH)^(n)/RMF/(HLH)^(n), or other similar configurations, where n=1-100and the L and H layers are 1 quarterwave (QW) at a design wavelength.

FIG. 3 depicts a reflective magnetic flake or particle (“RMP”) 60according to another embodiment of the invention. The RMP 60 is a twolayer design with a reflector layer 62 substantially surrounding andencapsulating a core magnetic layer 64. By inserting the magnetic layerwithin the reflector layer, the optical properties of the reflectorlayer are not downgraded and the reflector layer remains highlyreflective. The RMP 60 can be used as a pigment particle or can be usedas a core section with additional layers applied thereover. The magneticlayer and reflector layer can be composed of the same materialsdiscussed with respect to RMF 20.

In an alternative embodiment, a dielectric layer may optionally be addedto overlie reflector layer 62, to add durability, rigidity, andcorrosion resistance to RMP 60. The dielectric layer may be optionallyclear, or may be selectively absorbing so as to contribute to the coloreffect of the pigment flake.

FIG. 4 depicts alternative coating structures (with phantom lines) for amagnetic color shifting pigment flake 80 in the form of an encapsulatebased upon either the RMF or the RMP according to other embodiments ofthe invention. The flake 80 has a magnetic core section 82, which iseither a RMF or a RMP, which can be overcoated by an encapsulatingdielectric layer 84 substantially surrounding magnetic core section 82.An absorber layer 86, which overcoats dielectric layer 84, provides anouter encapsulation of flake 80. The hemispherical dashed lines on oneside of flake 80 in FIG. 4 indicate that dielectric layer 84 andabsorber layer 86 can be formed as contiguous layers around magneticcore section 82.

Alternatively, the magnetic core section 82 and dielectric layer can bein the form of a thin film core flake stack, in which opposingdielectric layers 84 a and 84 b are preformed on the top and bottomsurfaces but not on at least one side surface of magnetic core section82 (RMF), with absorber layer 86 encapsulating the thin film stack. Anencapsulation process can also be used to form additional layers onflake 80 such as a capping layer (not shown). The pigment flake 80exhibits a discrete color shift such that the pigment flake has a firstcolor at a first angle of incident light or viewing and a second colordifferent from the first color at a second angle of incident light orviewing.

In a further alternative embodiment, flake 80 can be formed without theabsorber layer. In this embodiment, dielectric layer 84 is formed ofcontiguous high/low (H/L) dielectric optical coatings similar to thedielectric optical stacks described previously. Thus, dielectric layer84 can have the coating structure (HL)^(n), (LH)^(n), (LHL)^(n),(HLH)^(n), or other similar configurations, where n=1-100 and the L andH layers are 1 QW at a design wavelength.

FIG. 5 depicts another alternative coating structure for a colorshifting pigment flake 100 according to the present invention. The flake100 includes a magnetic core section 82 and a single dielectric layer84, which extends over top and bottom surfaces of magnetic core section82 to form a dielectric-coated preflake 86. The core section 82 can bean RMF, RMP, or a magnetic layer. The dielectric-coated preflake 86 hastwo side surfaces 88 and 90. Although side surface 90 is homogeneous andformed only of the dielectric material of dielectric layer 84, sidesurface 88 has distinct surface regions 88 a, 88 b, 88 c of dielectric,magnetic core section, and dielectric, respectively. Thedielectric-coated preflake 86 is further coated on all sides with anabsorber layer 92. The absorber layer 92 is in contact with dielectriclayer 84 and magnetic core section 82 at side surface 88.

The structure of pigment flake 100 typically occurs because of apreflake coating process similar to the one disclosed in U.S.application Ser. No. 09/512,116 described previously. The preflakes canbe a dielectric-coated flake, in which a dielectric coating completelyencapsulates an RMF or RMP (see FIG. 4), or a magnetic layer (see FIG.10). The preflakes are broken into sized preflakes using anyconventional fragmentation process, such as by grinding. The sizedpreflakes will include some sized preflakes having top and bottomdielectric layers with no dielectric coating on the side surfaces of thepreflake, such as shown for the embodiment of flake 40 in FIG. 2 inwhich RMF 42 is coated with top and bottom dielectric layers 44 and 46.Other sized preflakes will have a single dielectric layer extending overboth top and bottom surfaces of the magnetic core flake section, leavingone side surface of the magnetic core flake section exposed, such asshown for dielectric-coated preflake 86 in FIG. 5. Because of thefragmentation process, substantially all of the sized preflakes have atleast a portion of a side surface exposed. The sized preflakes are thencoated on all sides with an absorber layer, such as shown in the flakesof FIGS. 4 and 5.

FIG. 6 depicts a composite magnetic flake (“CMF”) 120 which comprises acentral dielectric support layer 122 with first and second magneticlayers 124, 126 on opposing major surfaces thereof. By inserting thedielectric layer between the magnetic layers, the CMF 120 issignificantly stabilized and strengthened, having increased rigidity.Additional dielectric layers (not shown) may optionally be added tooverlie magnetic layers 124, 126. These additional dielectric layers adddurability, rigidity, and corrosion resistance to CMF 120. The CMF 120can be used as a pigment flake by itself or can be used as a magneticcore section with additional layers applied thereover. The magneticlayers 124, 126 can be formed of any of the magnetic materials describedpreviously.

The dielectric material used for support layer 122 is preferablyinorganic, since inorganic dielectric materials have been found to havegood characteristics of brittleness and rigidity. Various dielectricmaterials that can be utilized include metal fluorides, metal oxides,metal sulfides, metal nitrides, metal carbides, combinations thereof,and the like. The dielectric materials may be in either a crystalline,amorphous, or semicrystalline state. These materials are readilyavailable and easily applied by physical or chemical vapor depositionprocesses. Examples of suitable dielectric materials include magnesiumfluoride, silicon monoxide, silicon dioxide, aluminum oxide, titaniumdioxide, tungsten oxide, aluminum nitride, boron nitride, boron carbide,tungsten carbide, titanium carbide, titanium nitride, silicon nitride,zinc sulfide, glass flakes, diamond-like carbon, combinations thereof,and the like. Alternatively, support layer 122 may be composed of apreformed dielectric or ceramic preflake material having a high aspectratio such as a natural platelet mineral (e.g., mica peroskovite ortalc), or synthetic platelets formed from glass, alumina, silicondioxide, carbon, micaeous iron oxide, coated mica, boron nitride, boroncarbide, graphite, bismuth oxychloride, various combinations thereof,and the like.

In an alternative embodiment, instead of a dielectric support layer 122,various semiconductive and conductive materials having a sufficientratio of tensile to compressive strength can function as a supportlayer. Examples of such materials include silicon, metal silicides,semiconductive compounds formed from any of the group III, IV, or Velements, metals having a body centered cubic crystal structure, cermetcompositions or compounds, semiconductive glasses, various combinationsthereof, and the like. It will be appreciated from the teachings herein,however, that any support material providing the functionality describedherein and capable of acting as a rigid layer with glass-like qualitieswould be an acceptable substitute for one of these materials.

The thickness of support layer 122 can be in a range from about 10 nm toabout 1,000 nm, preferably from about 50 nm to about 200 nm, althoughthese ranges should not be taken as restrictive.

FIG. 7 depicts a composite magnetic particle (“CMP”) 140 according toanother embodiment of the invention. The CMP 140 is a two layer designwith a magnetic layer 142 substantially surrounding and encapsulating acentral support layer 144 such as a dielectric layer. By inserting thesupport layer within the magnetic layer, CMP 140 is significantlystabilized and rigid. The support layer adds rigidity and durability tothe pigment flake. The magnetic layer 142 can be formed of any of themagnetic materials described previously. The support layer 144 can beformed of the same materials described hereinabove for support layer 122of CMF 120. The CMP 140 can be used as a pigment particle by itself orcan be used as a magnetic core section with additional layers appliedthereover. For example, an outer dielectric layer may be added tooverlie and encapsulate magnetic layer 142. This outer dielectric layeradds durability, rigidity, and corrosion resistance to CMP 140.

FIG. 8 depicts a coating structure for a color shifting pigment flake160 in the form of an encapsulate. The flake 160 has a thin core layer162, which can be formed of a dielectric or other material as taughthereinabove for support layer 122. The core layer 162 is overcoated onall sides with a magnetic layer 164, which can be composed of the samematerials as described above for magnetic layer 22 of RMF 20.Optionally, a reflector layer 168 can be applied over magnetic layer164. Suitable materials for reflector layer 168 include those materialsdescribed for reflector layer 24 of RMF 20. The reflector layereffectively provides the reflective function of flake 160, shieldingmagnetic layer 164 from being optically present. The core layer 162 andmagnetic layer 164 can be provided as a CMP 166 which is overcoated withthe other layers. Alternatively CMP 166 can be replaced with a CMF suchas shown in FIG. 6. An encapsulating dielectric layer 170 substantiallysurrounds reflector layer 168 and magnetic layer 164. An absorber layer172, which overlays dielectric layer 170, provides an outerencapsulation of flake 160.

Various coating processes can be utilized in forming the dielectric andabsorber coating layers by encapsulation. For example, suitablepreferred methods for forming the dielectric layer include vacuum vapordeposition, sol-gel hydrolysis, CVD in a fluidized bed, downstreamplasma onto vibrating trays filled with particles, and electrochemicaldeposition. A suitable SiO₂ sol-gel process is described in U.S. Pat.No. 5,858,078 to Andes et al., the disclosure of which is incorporatedby reference herein. Other examples of suitable sol-gel coatingtechniques useful in the present invention are disclosed in U.S. Pat.No. 4,756,771 to Brodalla; Zink et al., Optical Probes and Properties ofAluminosilicate Glasses Prepared by the Sol-Gel Method, Polym. Mater.Sci. Eng., 61, pp. 204-208 (1989); and McKiernan et al., Luminescenceand Laser Action of Coumarin Dyes Doped in Silicate and AluminosilicateGlasses Prepared by the Sol-Gel Technique, J. Inorg. Organomet. Polym.,1(1), pp. 87-103 (1991); with the disclosures of each of theseincorporated by reference herein.

Suitable preferred methods for forming the absorber layers includevacuum vapor deposition, and sputtering onto a mechanically vibratingbed of particles, as disclosed in commonly assigned copending patentapplication Ser. No. 09/389,962, filed Sep. 3, 1999, entitled “Methodsand Apparatus for Producing Enhanced Interference Pigments,” which isincorporated by reference herein in its entirety. Alternatively, theabsorber coating may be deposited by decomposition through pyrolysis ofmetal-organo compounds or related CVD processes which may be carried outin a fluidized bed as described in U.S. Pat. Nos. 5,364,467 and5,763,086 to Schmid et al., the disclosures of which are incorporated byreference herein. If no further grinding is carried out, these methodsresult in an encapsulated core flake section with dielectric andabsorber materials therearound. Various combinations of the abovecoating processes may be utilized during manufacture of pigment flakeswith multiple encapsulating coatings.

In one method of forming the absorber coating, powdered flakes or othercoated preflakes are placed on a square-shaped vibrating conveyor coaterin a vacuum coating chamber as disclosed in U.S. application Ser. No.09/389,962, discussed above. The vibrating conveyor coater includesconveyor trays which are configured in an overlapping inclinedarrangement so that the powdered flakes travel along a circulating pathwithin the vacuum chamber. While the flakes circulate along this paththey are effectively mixed by constant agitation so that exposure to thevaporized absorber coating material is uniform. Efficient mixing alsooccurs at the end of each conveyor tray as the flakes drop in awaterfall off of one tray and onto the next tray. The absorber can besequentially coated on the flakes as they repeatably move under acoating material source.

When using vibrating conveyer trays to coat the absorber, it isimportant that the powdered flakes tumble randomly under the coatingmaterial source such as sputter targets and do not become subject to“metal welding” or sticking. Such metal welding or sticking can occurbetween two flat surfaces of reactive metals when such metals aredeposited in a vacuum. For example, aluminum has a high propensity tostick to itself, whereas chromium does not. Suitable absorber materialscan be applied as either a single material or as an outer capping layerover an underlying different absorber material.

FIG. 9 depicts a dielectric coated magnetic flake (“DMF”) 180 accordingto a further embodiment of the invention. The DMF 180 is a three layerdesign having a generally symmetrical thin film structure with a centralmagnetic layer and at least one dielectric layer on either or both ofthe opposing major surfaces of the central magnetic layer. Thus, asshown, DMF 180 includes a magnetic layer 182 sandwiched in between adielectric layer 184 and an opposing dielectric layer 186. By insertingthe magnetic layer between the dielectric layers, the DMF has increasedrigidity and durability.

FIG. 10 depicts a dielectric coated magnetic particle (“DMP”) 200according to another embodiment of the invention. The DMP 200 is a twolayer design with a dielectric layer 202 substantially surrounding andencapsulating a central magnetic layer 204.

Each of the layers in the coating structures of DMF 180 and DMP 200 canbe formed of the same materials and thickness as corresponding layersdescribed in previous embodiments. For example, the dielectric layer inDMF 180 and DMP 200 can be formed of the same materials and in the samethickness ranges as taught hereinabove for dielectric layer 44 of flake40, and the magnetic layers in DMF 180 and DMP 200 can be formed of thesame materials and in the same thickness ranges as taught hereinabovefor magnetic layer 22 of flake 20. The DMF 180 and DMP 200 can each beused as a pigment flake or particle, or can be used as a magnetic coresection with additional layers applied thereover.

FIG. 11 depicts a color shifting pigment flake 220 according to anotherembodiment of the invention which does not use a reflector (with highreflectance, i.e., an optical metal). The flake 220 is a three-layerdesign having a generally symmetrical multilayer thin film structure onopposing sides of a magnetic core section 222, which can be a DMF or aDMP. Thus, first and second absorber layers 224 a and 224 b are formedon opposing major surfaces of magnetic core section 222. These layers offlake 220 can be formed by a web coating and flake removal process asdescribed previously.

FIG. 11 further depicts an alternative coating structure (with phantomlines) for color shifting flake 220, in which the absorber layer iscoated around magnetic core section 222 in an encapsulation process.Accordingly, absorber layers 224 a and 224 b are formed as part of acontinuous coating layer 224 substantially surrounding the flakestructure thereunder.

Thus, pigment flake 220 may be embodied either as a multilayer thin filmstack flake or a multilayer thin film encapsulated particle. Suitablematerials and thicknesses for the absorber, dielectric, and magneticlayers of flake 220 are the same as taught hereinabove.

Some flakes of the invention can be characterized as multilayer thinfilm interference structures in which layers lie in parallel planes suchthat the flakes have first and second parallel planar outer surfaces andan edge thickness perpendicular to the first and second parallel planarouter surfaces. Such flakes are produced to have an aspect ratio of atleast about 2:1, and preferably about 5-15:1 with a narrow particle sizedistribution. The aspect ratio of the flakes is ascertained by takingthe ratio of the longest planar dimension of the first and second outersurfaces to the edge thickness dimension of the flakes.

One presently preferred method of fabricating a plurality of pigmentflakes, each of which having the multilayer thin film coating structureof flake 40 shown in FIG. 2, is based on conventional web coatingtechniques used to make optical thin films. Although flake 40 isdescribed hereinbelow, the other flake structures taught herein can alsobe fabricated with a procedure similar to the one described hereinbelow.Accordingly, a first absorber layer is deposited on a web of flexiblematerial such as polyethylene terephthalate (PET) which has an optionalrelease layer thereon. The absorber layer can be formed by aconventional deposition process such as PVD, CVD, PECVD, sputtering, orthe like. The above mentioned deposition methods enable the formation ofa discrete and uniform absorber layer of a desired thickness.

Next, a first dielectric layer is deposited on the absorber layer to adesired optical thickness by a conventional deposition process. Thedeposition of the dielectric layer can be accomplished by a vapordeposition process (e.g., PVD, CVD, PECVD), which results in thedielectric layer cracking under the stresses imposed as the dielectrictransitions from the vapor into the solid phase.

The magnetic core is then deposited. In the case of reflector layers, afirst reflector layer is then deposited by PVD, CVD, or PECVD on thefirst dielectric layer, taking on the characteristics of the underlyingcracked dielectric layer. Magnetic layers are then applied by e-beamevaporation, sputtering, electrodeposition, or CVD, followed by a secondreflector layer being deposited.

This is followed by a second dielectric layer being deposited on thesecond reflector layer and preferably having the same optical thicknessas the first dielectric layer. Finally, a second absorber layer isdeposited on the second dielectric layer and preferably has the samephysical thickness as the first absorber layer.

Thereafter, the flexible web is removed, either by dissolution in apreselected liquid or by way of a release layer, both of which are wellknown to those skilled in the art. As a result, a plurality of flakesare fractured out along the cracks of the layers during removal of theweb from the multilayer thin film. This method of manufacturing pigmentflakes is similar to that more fully described in U.S. Pat. No.5,135,812 to Phillips et al., the disclosure of which is incorporated byreference herein. The pigment flakes can be further fragmented ifdesired by, for example, grinding the flakes to a desired size using anair grind, such that each of the pigment flakes has a dimension on anysurface thereof ranging from about 2 microns to about 200 microns.

In order to impart additional durability to the color shifting flakes,an annealing process can be employed to heat treat the flakes at atemperature ranging from about 200-300° C., and preferably from about250-275° C., for a time period ranging from about 10 minutes to about 24hours, and preferably a time period of about 15-60 minutes.

Other pigment flake structures, methods of forming them, and additionalfeatures compatible therewith can be found in Phillips '648, U.S. Pat.No. 4,705,356 to Berning et al., and U.S. Pat. No. 6,157,489 to Bradleyet al.; U.S. patent application Ser. Nos. 09/685,468 to Phillips et al,09/715,937 to Coombs et al., 09/715,934 to Mayer et al., 09/389,962 toPhillips et al., and 09/539,695 to Phillips et al., the disclosures ofwhich are each incorporated herein by reference. One skilled in the artwill recognize, in light of the disclosure herein, that the magneticlayers discussed previously can be combined with the coating structuresdisclosed in the above patents and applications, such as by replacing areflector layer with the RMF or RMP disclosed herein to obtainadditional useful coating structures.

Referring now to FIG. 12, pigment flake 240 is deposited according toanother embodiment of the invention. As illustrated, flake 240 is amultilayer design having a generally symmetrical thin film structure onopposing sides of a magnetic layer such as a reflective magnetic core242, which can be any non-color shifting magnetic pigment flake orparticle having reflective properties described herein or known in theart. For example, reflective magnetic core 242 can be a singlereflective magnetic layer such as a monolithic layer of Ni or othermagnetic reflective metal, or can be a multilayer magnetic structuresuch as Al/Fe/Al. A first colored layer such as selective absorber layer244 a and a second colored layer such as selective absorber layer 244 bare formed on opposing major surfaces of reflective magnetic core 242.These colored layers of flake 240 can be formed by a web coating andflake removal process as described previously.

FIG. 12 further depicts an alternative coating structure (with phantomlines) for flake 240, in which a colored layer such as selectiveabsorber layer 244 is coated around reflective magnetic core 242 in anencapsulation process. Accordingly, selective absorber layers 244 a and244 b are formed as part of a contiguous coating layer 244 substantiallysurrounding the flake structure thereunder. Suitable encapsulationmethods for forming flake 240 are as described in a copending U.S.application Ser. No. 09/626,041, filed Jul. 27, 2000, the disclosure ofwhich is incorporated by reference herein.

Thus, pigment flake 240 may be embodied either as a multilayer thin filmstack flake or a multilayer thin film encapsulated particle. Suitablematerials and thicknesses for use in the reflective magnetic core offlake 240 are the same as taught hereinabove, so long as both reflectiveand magnetic properties are maintained.

The colored layers of flake 240 can be formed of a variety of differentabsorbing and/or reflecting materials in one or more layers. Preferably,the colored layers such as selective absorber layers are formed to havea thickness of from about 0.05 μm to about 5 μm, and more preferablyfrom about 1 μm to about 2 μm, by conventional coating processes for dyestuffs, when an organic dye material is utilized to form the selectiveabsorber layers. Preferably, the colored layers are formed to have athickness of from about 0.05 μm to about 0.10 μm when colored metallicsor other inorganic colorant materials are utilized.

Examples of suitable organic dyes that can be used to form the selectiveabsorber layers of flake 240 include copper phthalocyanine,perylene-based dyes, anthraquinone-based dyes, and the like; azo dyesand azo metal dyes such as aluminum red (RLW), aluminum copper, aluminumbordeaux (RL), aluminum fire-red (ML), aluminum red (GLW), aluminumviolet (CLW), and the like; as well as combinations or mixtures thereof.Such dyes can be applied by conventional coating techniques and even byevaporation.

The colored layers of flake 240 can also be formed of a variety ofconventional organic or inorganic pigments applied singly or dispersedin a pigment vehicle. Such pigments are described in the NPIRI RawMaterials Data Handbook, Vol. 4, Pigments (1983), the disclosure ofwhich is incorporated by reference herein.

In another embodiment, the selective absorber layers of flake 240comprise a sol-gel matrix holding a colored pigment or dye. For example,the selective absorber layer can be formed of aluminum oxide or silicondioxide applied by a sol-gel process, with organic dyes absorbed intopores of the sol-gel coating or bound to the surface of the coating.Suitable organic dyes used in the sol-gel coating process include thoseavailable under the trade designations Aluminiumrot GLW (aluminum redGLW) and Aluminiumviolett CLW (aluminum violet CLW) from the SandozCompany. Aluminum red GLW is an azo metal complex containing copper, andaluminum violet CLW is a purely organic azo dye. Examples of sol-gelcoating techniques useful in the present invention are disclosed in thefollowing: U.S. Pat. No. 4,756,771 to Brodalla (1988); Zink et al.,Optical Probes and Properties of Aluminosilicate Glasses Prepared by theSol-Gel Method, Polym. Mater. Sci. Eng., 61, pp. 204-208 (1989); andMcKiernan et al., Luminescence and Laser Action of Coumarin Dyes Dopedin Silicate and Aluminosilicate Glasses Prepared by the Sol-GelTechnique, J. Inorg. Organomet. Polym., 1(1), pp. 87-103 (1991); thedisclosures of all of these are incorporated herein by reference.

In a further embodiment, the colored layers of flake 240 can be formedof an inorganic colorant material. Suitable inorganic colorants includeselective absorbers such as titanium nitride, chromium nitride, chromiumoxide, iron oxide, cobalt-doped alumina, and the like, as well ascolored metallics such as copper, brass, titanium, and the like.

It should be understood that various combinations of the above dyes,pigments, and colorants may also be employed to achieve a desired colorcharacteristic for flake 240. The organic dyes, pigments, and colorantsdiscussed herein can be used in the invention to achieve pigments withbright colors having magnetic properties.

Various modifications and combinations of the foregoing embodiments arealso considered within the scope of the invention. For example,additional dielectric, absorber, and/or other optical coatings can beformed around each of the above flake or particle embodiments, or on acomposite reflective film prior to flake formation, to yield furtherdesired optical characteristics. Such additional coatings can provideadditional color effects to the pigments. For example a coloreddielectric coating added to a color shifting flake would act as a colorfilter on the flake, providing a subtractive color effect which changesthe color produced by the flake.

The pigment flakes of the present invention can be interspersed within apigment medium to produce a colorant composition which can be applied toa wide variety of objects or papers. The pigment flakes added to amedium produces a predetermined optical response through radiationincident on a surface of the solidified medium. Preferably, the pigmentmedium contains a resin or mixture of resins which can be dried orhardened by thermal processes such as thermal cross-linking, thermalsetting, or thermal solvent evaporation or by photochemicalcross-linking. Useful pigment media include various polymericcompositions or organic binders such as alkyd resins, polyester resins,acrylic resins, polyurethane resins, vinyl resins, epoxies, styrenes,and the like. Suitable examples of these resins include melamine,acrylates such as methyl methacrylate, ABS resins, ink and paintformulations based on alkyd resins, and various mixtures thereof. Theflakes combined with the pigment media produce a colorant compositionthat can be used directly as a paint, ink, or moldable plastic material.The colorant composition can also be utilized as an additive toconventional paint, ink, or plastic materials.

The pigment medium also preferably contains a solvent for the resin. Forthe solvent, generally, either an organic solvent or water can be used.A volatile solvent can also be used in the medium. As for the volatilesolvent, it is preferable to use a solvent which is both volatile aswell as dilutable, such as a thinner. In particular, faster drying ofthe pigment medium can be achieved by increasing the amount of thesolvent with a low boiling point composition such as methyl ethyl ketone(MEK).

In addition, the flakes can be optionally blended with various additivematerials such as conventional pigment flakes, particles, or dyes ofdifferent hues, chroma and brightness to achieve the colorcharacteristics desired. For example, the flakes can be mixed with otherconventional pigments, either of the interference type ornoninterference type, to produce a range of other colors. Thispreblended composition can then be dispersed into a polymeric mediumsuch as a paint, ink, plastic or other polymeric pigment vehicle for usein a conventional manner.

Examples of suitable additive materials that can be combined with theflakes of the invention include non-color shifting high chroma or highreflective platelets which produce unique color effects, such asMgF₂/Al/MgF₂ platelets, or SiO₂/Al/SiO₂ platelets. Other suitableadditives that can be mixed with the magnetic color shifting flakesinclude lamellar pigments such as multi-layer color shifting flakes,aluminum flakes, graphite flakes, glass flakes, iron oxide, boronnitride, mica flakes, interference based TiO₂ coated mica flakes,interference pigments based on multiple coated plate-like silicaticsubstrates, metal-dielectric or all-dielectric interference pigments,and the like; and non-lamellar pigments such as aluminum powder, carbonblack, ultramarine blue, cobalt based pigments, organic pigments ordyes, rutile or spinel based inorganic pigments, naturally occurringpigments, inorganic pigments such as titanium dioxide, talc, china clay,and the like; as well as various mixtures thereof. For example, pigmentssuch as aluminum powder or carbon black can be added to controllightness and other color properties.

The magnetic color shifting flakes of the present invention areparticularly suited for use in applications where colorants of highchroma and durability are desired. By using the magnetic color shiftingflakes in a colorant composition, high chroma durable paint or ink canbe produced in which variable color effects are noticeable to the humaneye. The color shifting flakes of the invention have a wide range ofcolor shifting properties, including large shifts in chroma (degree ofcolor purity) and also large shifts in hue (relative color) with avarying angle of view. Thus, an object colored with a paint containingthe color shifting flakes of the invention will change color dependingupon variations in the viewing angle or the angle of the object relativeto the viewing eye.

The pigment flakes of the invention can be easily and economicallyutilized in paints and inks which can be applied to various objects orpapers, such as motorized vehicles, currency and security documents,household appliances, architectural structures, flooring, fabrics,sporting goods, electronic packaging/housing, product packaging, etc.The color shifting flakes can also be utilized in forming coloredplastic materials, coating compositions, extrusions, electrostaticcoatings, glass, and ceramic materials.

Generally, the foils of the invention have a nonsymmetrical thin filmcoating structure, which can correspond to the layer structures on oneside of an RMF in any of the above described embodiments related to thinfilm stack flakes. The foils can be laminated to various objects or canbe formed on a carrier substrate. The foils of the invention can also beused in a hot stamping configuration where the thin film stack of thefoil is removed from a release layer of a substrate by use of a heatactivated adhesive and applied to a countersurface. The adhesive can beeither coated on a surface of the foil opposite from the substrate, orcan be applied in the form of a UV activated adhesive to the surface onwhich the foil will be affixed.

FIG. 13 depicts a coating structure of a color shifting foil 300 formedon a substrate 302, which can be any suitable material such as aflexible PET web, carrier substrate, or other plastic material. Asuitable thickness for substrate 302 is, for example, about 2 to 7 mils.The foil 300 includes a magnetic layer 304 on substrate 302, a reflectorlayer 306 on magnetic layer 304, a dielectric layer 308 on reflectorlayer 306, and an absorber layer 310 on dielectric layer 308. Themagnetic, reflector, dielectric and absorber layers can be composed ofthe same materials and can have the same thicknesses as described abovefor the corresponding layers in flakes 20 and 40.

The foil 300 can be formed by a web coating process, with the variouslayers as described above sequentially deposited on a web byconventional deposition techniques to form a thin film foil structure.The foil 300 can be formed on a release layer of a web so that the foilcan be subsequently removed and attached to a surface of an object. Thefoil 300 can also be formed on a carrier substrate, which can be a webwithout a release layer.

FIG. 14 illustrates one embodiment of a foil 320 disposed on a web 322having an optional release layer 324 on which is deposited a magneticlayer 326, a reflector layer 328, a dielectric layer 330, and anabsorber layer 332. The foil 320 may be utilized attached to web 322 asa carrier when a release layer is not employed. Alternatively, foil 320may be laminated to a transparent substrate (not shown) via an optionaladhesive layer 334, such as a transparent adhesive or ultraviolet (UV)curable adhesive, when the release layer is used. The adhesive layer 334is applied to absorber layer 332.

FIG. 15 depicts an alternative embodiment in which a foil 340 having thesame thin film layers as foil 320 is disposed on a web 322 having anoptional release layer 324. The foil 340 is formed such that absorberlayer 332 is deposited on web 322. The foil 340 may be utilized attachedto web 322 as a carrier, which is preferably transparent, when a releaselayer is not employed. The foil 340 may also be attached to a substratesuch as a countersurface 342 when the release layer is used, via anadhesive layer 334 such as a hot stampable adhesive, a pressuresensitive adhesive, a permanent adhesive, and the like. The adhesivelayer 334 can be applied to magnetic layer 326 and/or to countersurface342.

When a hot stamp application is employed, the optical stack of the foilis arranged so that the optically exterior surface is adjacent therelease layer. Thus, for example, when foil 340 in FIG. 15 is releasedfrom web 322, absorber layer 332 is optically present on the exterior.In one preferred embodiment, release layer 324 is a transparent hardcoatthat stays on absorber layer 332 to protect the underlying layers aftertransfer from web 322.

Further details of making and using optical stacks as hot stamping foilscan be found in U.S. Pat. Nos. 5,648,165, 5,002,312, 4,930,866,4,838,648, 4,779,898, and 4,705,300, the disclosures of which areincorporated by reference herein.

Referring now to FIG. 16, another embodiment of the invention isdepicted in the form of an optical article 400 having paired opticalstructures. The optical article 400 includes a substrate 402 having anupper surface 404 and a lower surface 406. The substrate 402 can beflexible or rigid and can be formed of any suitable material such aspaper, plastic, cardboard, metal, or the like, and can be opaque ortransparent. Non-overlapping paired first and second coating structures408, 410 are disposed on upper surface 404 so as to overlienon-overlapping first and second regions on surface 404. Thus, first andsecond coating structures 408, 410 are not superimposed but arephysically separated from each other on surface 404, although in anabutting relationship. For example, in one embodiment, first coatingstructure 408 can be in the form of a rectangle or square and isdisposed within a recess 412 formed by second coating structure 410,also being in the form of a rectangle or square that forms a border orframe that surrounds first coating structure 408. Thus, when opticalarticle 400 is viewed from above, coating structures 408, 410 can beviewed simultaneously.

The first coating structure 408 has a first pigment 414 formed ofmagnetic pigment flakes or particles, such as color shifting magneticflakes, constructed in the manner hereinbefore described to provide amagnetic signature. The magnetic properties of pigment 414 are providedby a non-optically observable magnetic layer within one or more of themagnetic flakes or particles. The second coating structure 410 has asecond pigment 416 formed of non-magnetic pigment flakes or particles,such as color shifting non-magnetic flakes. Alternatively, the secondcoating structure 410 could be formed to contain the magnetic pigmentsand the first coating structure 408 could be formed to contain thenon-magnetic pigments. The pigments 414, 416 are dispersed in asolidified liquid pigment vehicle 418, 420 of a conventional type sothat the pigments 414, 416 produce the desired optical characteristics.For example, the liquid vehicle can be a conventional ink vehicle or aconventional paint vehicle of a suitable type.

In an alternative embodiment, optical article 400 can be formed by usinga suitable magnetic foil structure, such as the color shifting magneticfoils disclosed hereinabove, in place of coating structure 408, and byusing a non-magnetic foil structure such as a conventional colorshifting foil in place of coating structure 410. The magnetic propertiesof the magnetic foil structure are thus provided by a magnetic layerwhich is not optically observable. Non-overlapping paired first andsecond foil structures, one magnetic and one non-magnetic, would bedisposed on upper surface 404 of substrate 402 so as to overlienon-overlapping first and second regions on surface 404.

Other optical articles with paired optically variable structures, whichcould be modified to include magnetic layers in one of the pairedstructures such as disclosed herein, are taught in U.S. Pat. No.5,766,738 to Phillips et al., the disclosure of which is incorporated byreference herein.

Referring now to FIG. 17, another embodiment of the invention isdepicted in the form of an optical article 450 having overlapping pairedoptical structures. The optical article 450 includes a substrate 452having an upper surface region 454. The substrate 452 can be formed ofthe same materials as described for substrate 402 shown in FIG. 16. Amagnetic pigment coating structure 456 overlies upper surface region 454of substrate 452. The magnetic pigment coating structure 456 includes aplurality of multilayer magnetic pigments 458, such as those describedpreviously, which are dispersed in a solidified pigment vehicle. Themagnetic properties of the pigment coating structure 456 are provided bya non-optically observable magnetic layer within each of the multilayermagnetic pigments 458. A non-magnetic pigment coating structure 460overlies at least a portion of magnetic pigment coating structure 456.The non-magnetic pigment coating structure 460 includes a plurality ofnon-magnetic pigments 462 dispersed in a solidified pigment vehicle.

In an alternative embodiment of optical article 450, a non-magneticpigment coating structure can be used in place of magnetic pigmentcoating structure 456 overlying upper surface region 454 of substrate452. A magnetic pigment coating structure is then used in place ofnon-magnetic pigment coating structure 460.

In a further alternative embodiment, optical article 450 can be formedby using a suitable magnetic foil structure, such as the color shiftingmagnetic foils disclosed hereinabove, in place of coating structure 456.A non-magnetic foil structure such as a conventional color shifting foilis then used in place of coating structure 460. Alternatively, anon-magnetic foil structure can be used in place of coating structure456, and a magnetic foil structure is then used in place of coatingstructure 460.

The respective pigment coating or foil structures in optical articles400 or 450 can be selected to provide identical coloring or identicalcolor shifting effects to articles 400 and 450, or can be selected toprovide different colors or different color shifting effects. Of course,one skilled in the art will recognize that a variety of combinations ofoptical features can be used by selecting appropriate coatings or foilswith the desired optical characteristics to add various securityfeatures to optical articles 400 and 450.

Although the pigment coating or foil structures used in articles 400 and450 may have substantially the same color or color effects, e.g., thesame color shifting effects, only one of the pigment coating or foilstructures in the articles carries a covert magnetic signature.Therefore, although a human eye cannot detect the magnetic features ofthe pigment coating or foil structure, a magnetic detection system suchas a Faraday rotator detector can be used to detect the magnetic covertsignature in the pigment or foil and any information magneticallyencoded therein.

From the foregoing it can be seen that there have been provided thinfilm structures which have both magnetic, and optionally, color shiftingproperties, which have many different types of applications,particularly where additional security is desired.

For example, a structure or device formed with the pigments of theinvention can be placed in a bar code pattern which would produce acolor shifting bar code device that can appear on a label or on anarticle itself. Such a bar code would function as a color shifting barcode that could be read by both optical and magnetic readers. Such a barcode color shifting device would provide three security features, thebar code itself, the color shifting characteristic, and the magneticcharacteristic. In addition, information can be encoded in the magneticlayers of the pigments of the invention. For example, the magneticlayers could record typical information which is carried by a creditcard in a magnetic stripe. In addition, pigments of the invention couldbe utilized for putting the numbers on the bottoms of checks so that theinformation carried by the check could be read magnetically as withpresent day checks while also providing an optical variable feature.

The following examples are given to illustrate the present invention,and are not intended to limit the scope of the invention.

EXAMPLE 1

A three layer magnetic coating sample was prepared with 1000 Å Aluminum,1000 Å Iron, and 1000 Å Aluminum (Al/Fe/Al). The coating sample wasprepared in a roll coater, using a 2 mil polyester web coated with anorganic release layer (soluble in acetone). After stripping the threelayer coating from the web to form pigment flake particles, theparticles were filtered and sized by exposing the particles in isopropylalcohol to ultrasonic agitation for 5 minutes using a Branson sonicwelder. Particle size was determined using a Horiba LA-300 particlesizing instrument (laser scattering based system). The mean particlesize was determined to be 44 μm (22 μm standard deviation) in the planardimension, with a gaussian distribution. Following the sizing, thepigment particles were filtered and dried.

A dry weight of magnetic pigment to binder (Du Pont auto refinish paintvehicle) in the ratio of 1:4 was drawn down onto a thin cardboard sheet(Leneta card). A “draw-down” is a paint or ink sample spread on paper toevaluate the color. Typically, a draw-down is formed with the edge of aputty knife or spatula by “drawing down” a small glob of paint or ink toget a thin film of the paint or ink. Alternatively, the draw-down ismade using a Mayer rod pulled across a Leneta card and through a smallglob of paint. A conventional sheet magnet was placed underneath thecard while the drawing down was occurring and left in place until thepaint vehicle dried. The result of the magnetic fields on this pigmentsample was to create parallel bright and dark areas in the pigment. Byusing an ultra small area viewer (USAV, 2.3 mm) on a SF-600 DataColorspectrophotometer, the bright aluminum areas of the pigment sample had areflective luminance, Y, of 53% whereas the dark areas had a reflectiveluminance of 43%. However, it was difficult to fit the aperture withinthe dark and bright lines suggesting that the difference in brightnessmay actually be larger than these measurements.

EXAMPLE 2

A magnetic ink sample was prepared by mixing a 0.5 g sample of themagnetic pigment of Example 1 (Al/Fe/Al) with 3.575 g of standardIntaglio ink vehicle (high viscosity ink vehicle) and 0.175 g of an inkdryer. The ink sample was drawn down onto paper using a flat puttyknife. A magnetic strip with the word “FLEX” cut out from it was placedbeneath the paper during the drawing down step. The pattern of themagnetic lines in the dried magnetic ink was readily visible as blackand white (silver color) strips with the word “FLEX” readily apparent.The optical image of the word “FLEX” in the ink sample was visible atnormal incidence and at approximately a 45 degree angle of viewing.

EXAMPLE 3

A magnetic ink sample was prepared as in Example 2 using an Intaglio inkvehicle and coated over paper having a sheet magnet placed behind it.The magnet had a cut out of a stylized letter “F.” In addition to themagnetic pigment (Al/Fe/Al) orienting along the magnetic field lines,the cut out “F” was embossed upward away from the paper and was brightsilver in appearance. The “F” stood out over the surrounding area byabout 6 microns. This was caused by the paper pushed slightly into the“F” recess of the magnet by the force of the putty knife drawing downthe highly viscous Intaglio ink. Alter the paper relaxed, the “F” arearemained bright with the Al/Fe/Al flakes oriented parallel to thesurface of the paper but in a stepped-up height above the surroundingcoating.

EXAMPLE 4

A stylized letter “F” was cut out of a flexible sheet magnet using anexacto knife. A draw-down card was placed on top of and in contact withthe sheet magnet. A magnetic color shifting pigment according to theinvention was mixed with an acrylic resin based vehicle and applied tothe card with a #22 wire metering rod. The resultant draw-down hadstriped superimposed black lines that replicated the field patternoutside of the stylized “F” in the sheet magnet below the card. Theentire surface of the drawn-down card exhibited color shifting effects.Where the pattern of the stylized “F” was observed, the stylized “F”only had color shifting effects, while the background had both colorshifting effects and the superimposed black lines.

The cut out stylized letter “F” pieces from the sheet magnet were usedin another draw-down with the same magnetic pigment and vehicledescribed previously in this example. The resultant draw-down hadstriped superimposed black lines that replicated the field patternwithin the cutout stylized “F” magnet pieces. The entire surface of thedrawn-down exhibited a color shifting effect. Where the pattern of thestylized “F” was observed, the stylized “F” had both color shiftingeffects and the superimposed black lines, while the background had onlycolor shifting effects.

Thus, in both instances the entire surface of the draw-down cardsexhibited color shifting effects, while the areas directly above themagnets additionally had superimposed striped black lines due to themagnetic field pattern.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. A magnetic pigment flake, comprising: a central magneticlayer having a first major surface, an opposing second major surface,and at least one side surface; a first dielectric layer on the firstmajor surface of the magnetic layer; and a second dielectric layer onthe second major surface of the magnetic layer; wherein the dielectriclayers provide increased rigidity, durability, and corrosion resistanceto the pigment flake, with the pigment flake exhibiting magneticcharacteristics based on the relative magnetism of the magnetic layer.2. The pigment flake of claim 1, wherein the first and second dielectriclayers are selectively absorbing and provide additional color effects tothe pigment flake.
 3. The pigment flake of claim 1, wherein the magneticlayer comprises a soft magnetic material.
 4. The pigment flake of claim1, wherein the magnetic layer is composed of a material with acoercivity of less than about 2000 Oe.
 5. The pigment flake of claim 1,wherein the first and second dielectric layers are on each of the firstand second major surfaces but not on the at least one side surface ofthe magnetic layer.
 6. The pigment flake of claim 1, wherein the firstand second dielectric layers form part of a contiguous dielectric layersubstantially surrounding the magnetic layer.
 7. The pigment flake ofclaim 6, wherein the contiguous dielectric layer is selectivelyabsorbing and provides additional color effects to the pigment flake. 8.The pigment flake of claim 6, further comprising an absorber layersubstantially surrounding the flake.